The so-called interfacial polymerization method in which a hydrophobic substance is dispersed in an aqueous medium and an organic high-molecular substance (polymer) is allowed to grow at the interfaces between the dispersed droplets and the aqueous medium thereby to form minute particles is already well known.
In most of such techniques, an isocyanate prepolymer containing an isocyanate end group is dispersed in water and a polyamine or the like is added to the resulting dispersion, thereby to form stable polyurethane polyurea particles.
A feature of such interfacial polymerization reactions resides in that one of the particle wall-forming ingredients is fed only from the outside of the droplets. However, this feature has been a defect of interfacial polymerization reactions of the above kind.
Illustratively stated, according to the conventional technique, particle walls are formed by a polyurea-forming reaction between an amine and an isocyanate and once particle wall formation has been accomplished by this polyurea-forming reaction, the isocyanate groups remaining inside the particle walls are insulated by the walls from amino groups present in the water and a further reaction is not able to readily proceed. For this reason, as the particle walls grow further, migration of the amine from the water into the particles becomes significantly slow and, as a result, the reaction apparently comes to equilibrium, with unreacted isocyanate groups remaining undesirably within the particles.
The conventional interfacial polymerization technique, therefore, is defective in that the reproducibility of basic particle properties is poor and there also is a problem that reactive isocyanate groups undesirably remain or accumulate in the particles.
As an expedient for improving the shape of polyurethane polyurea particles, there is a method of conducting interfacial polymerization reaction in which a dispersant selected from various kinds is used when an isocyanate prepolymer having an isocyanate end group is dispersed in water, thereby to obtain spherical polyurethane polyurea particles. However, this method is defective in that it is necessary to use a dispersant in a large amount in order to obtain truly spherical particles and the large dispersant amount not only results in impaired surface properties of the resulting polyurethane polyurea particles due to the dispersant remaining on the particle surface, but also poses production problems such as the treatment of wastewater containing the dispersant.
Although polyurethane polyurea particles generally have good mechanical properties, chemical resistance, and other properties and are hence being utilized in various fields, they have a defect that, according to the shape of the particles, they cannot fully exhibit their intrinsic properties. Therefore, the production of polyurethane polyurea particles having a spherical shape like a truly spherical shape and no reactive isocyanate group are desired.